1. Field of the Invention
The present invention relates broadly to air chambers adapted to support an external load and an apparatus for maintaining the air pressure within the chamber at a relatively constant preset level when the load is applied thereto. In particular, the invention relates to a constant pressure chamber suitable for use as an air mattress, seat cushion or other load bearing device.
2. Description of Prior Art
A long-standing problem in air mattress design involves the ability to maintain constant pressure within the air mattress in spite of varying loads on the mattress as one or more users sit or lie on the bed, toss and turn during sleep or arise and return to bed. The "feel" of the mattress to the user is directly related to the amount of air pressure within the mattress. Thus, in an air mattress into which air is drawn to a certain pressure and then sealed, such as those shown in U.S. Pat. Nos. 3,872,525 and 3,877,092, the internal pressure increases when a user lies thereon and the mattress thus "feels" harder as the load thereon increases.
U.S. Pat. Nos. 4,224,706 and 4,306,322 disclose air mattress systems which allow the firmness of the mattress to be controlled when a person is lying thereon through the use of a separate bladder which contains a quantity of air adapted to be transferred between the air mattress and the bladder responsive to changes in the volume of the bladder.
Also known are manually operated systems, such as that shown in U.S. Pat. No. 4,394,784, where air is supplied to a mattress by a blower or vented from the mattress through valves, both the blower and the valves being electronically controlled by hand-held control units. U.S. Pat. No. 4,078,842 discloses an inflatable auto seat wherein pressurized air is supplied by a manually operated compressor. U.S. Pat. No. 3,303,518 discloses an inflatable mattress wherein air is supplied to compartments therein by hoses connected to a remotely located compressor/pump controlled by the user.
U.S. Pat. No. 4,686,722 discloses a mattress formed from a plurality of individual cushions interconnected by ducting to an air pressure source. The pressure in selected cushions may be controlled by computer.
U.S. Pat. No. 4,694,520 discloses an air mattress which includes a sensor 170 positioned within the mattress which turns on an air compressor when the mattress deflates to a point where a patient comes in contact therewith.
U.S. Pat. No. 4,711,275 discloses a mattress including a rather complex system having a plurality of air compressors and pressure sensors to inflate and deflate portions of the mattress in cycle to prevent bedsores on a bedridden patient.
U.S. Pat. No. 4,679,264 discloses a self-regulating air mattress including a reservoir and means for adding or removing air from the system. A sensing device is disclosed which is adapted to sense the pressure in the mattress and add or remove air therefrom to maintain a constant pressure. Experimentation has shown, however, that such a system, supposedly designed to maintain pressure within a mattress at a predetermined level by sensing pressure and adding or removing air from the mattress in response to a change in pressure, simply does not work. The problem of such systems is that, assuming a preset pressure to be sensed and maintained, the pressure within the mattress is increased when a load is placed thereon. This increased pressure is sensed and air is vented from the mattress in response thereto. However, venting of air from the mattress does not decrease pressure within the mattress so long as the load remains thereon until the mattress is almost totally deflated.
The present invention provides an air mattress or the like which can be maintained at a constant pressure even under load without deflating.